From the author of

From the author of

Scripting the Playhead

Just as you generated the grid dynamically earlier, you will also use
ActionScript to attach the playhead dynamically so that it is always positioned
relative to the size of your variable-defined grid.

You'll begin with the playHead movie clip, which consists of an
upper arrow (graphic), a center bar (bar movie clip), and a bottom arrow
(knob movie clip). The challenge comes with sizing the bar to match the
grid. You will accomplish this by scaling the bar to the height of your
movieGrid container and placing the bottom arrow below that.

Double-click on the playHead movie clip in the Library and enter
Edit mode. See Figure 10.